1. Field of the Invention
The present invention relates to a method for producing an ester. More specifically, the present invention relates to a method for producing an ester that can provide a high-quality and heat-resistant ester with high yield.
2. Description of the Prior Art
Esters are used in a wide range of fields such as cosmetics, pharmaceutical preparations, foods, electronic equipment, printing, and lubricants, to mention a few. In recent years, with technological development in these fields, there is a large demand for high-quality esters. For example, for ester waxes having a high melting point used in lubricants for toner, sharp melting characteristics and high heat resistance are required. Therefore, for this use, there is a demand for an ester having a small content of low volatile substances, raw material alcohols, raw material carboxylic acids, esters containing a hydroxyl group or the like, and which exhibits a small weight loss at high temperatures. For esters used for refrigerating machine oils, such properties as high electric insulation and a high heat resistance are required. In order o obtain such properties, it is necessary that very low levels of contaminants or conductive impurities are present and the acid value and hydroxyl value of the esters are low, in order that the hydrolytic stability and heat stability at high temperatures of the esters are excellent.
Esters can be produced by a reaction between an alcohol and a carboxylic acid, as well known. In general, this reaction is carried out with an excessive amount of carboxylic acid, and an ester can be obtained from the resultant crude esterified product by a process of removing the excessive carboxylic acid by one of the following processes alone or by combining these processes: a process of topping under reduced pressure; a process of neutralization with an alkaline aqueous solution; and a process of adsorption treatment that removes the carboxylic acid by adsorption. However, if the process of topping under reduced pressure is employed alone, the acid value is insufficiently reduced, and furthermore, this process may lead to heat deterioration because of exposure to high temperature, which may impair the quality.
Therefore, neutralization with an alkali aqueous solution is commonly employed after an esterification reaction, as disclosed in the examples of Japanese Laid-Open Patent Publication Nos. 6-271881, 7-118681 and 9-316479. However, neutralization has the problems that a produced ester is incorporated into an alkaline aqueous layer, which results in poor separation of the layers. In particular, in the case of a highly viscous ester or a high melting point ester derived from straight and long chain saturated monocarboxylic acid, the layers are poorly separated or an emulsified state occurs, so that the acid value is not sufficiently reduced or the yield is significantly decreased. To solve these problems, the following processes are employed: a process of adding hot water in which a salt such as sodium chloride or sodium sulfate is dissolved to a mixture to be treated for neutralization so as to increase the difference in the relative density between the ester layer and the alkaline aqueous layer, so that the separation can be improved; and a process of dehydrating a mixture containing an emulsified ester layer and alkaline layer under reduced pressure as it is, and then filtrating the mixture. However, these processes cannot provide sufficient improvement effects, and therefore, cause new problems in that the added salt or a formed carboxylic acid soap (salt of carboxylic acid) remain in the ester, which significantly degrades the quality of the ester.
In addition, purification by performing an absorption treatment with an adsorbent such as activated clay, silica gel, acid clay and a silica-alumina synthetic adsorbent is also used. For example, the examples of Japanese Laid-Open Patent Publication No. 11-80766 describes a purification process by an adsorption treatment, but cannot provide an ester having a satisfactory quality in terms of heat stability and oxidation stability.